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Enhanced Gas Recovery Techniques From Coalbed Methane Reservoirs 235

these reservoirs differ to some extent. The most significant parameters in coals, playing
key roles in reservoir studies, consist of coal rank, macerals, permeability, porosity,
density, rock mechanical properties, and sorption properties. Therefore, in this section
these features are briefly introduced.

8.2.1 Coal Rank

One of the main aspects of coals’ classification is their rank, defined as the degree to
which the coal is thermally mature or in other words, the degree of metamorphosis
of the existing organic materials in the coal. Coal rank is one of the main criteria for
gas content determination, because this factor indicates the adsorption capacity of the
rocks. Coals are organic rocks that corresponding to their geological age, the level of
impurity, and moisture content, reveal distinct complexities in different reservoirs. In
fact, deeper coals are generally more mature, and due to subjection to more heat and
pressure, these coals have expelled most of the water from the rock texture, resulting
in a more mechanically integrated rock with its surface covered with longer chained
gases as well as aromatic molecules. On the contrary, more recent coals with less burial
depth are of lower rank and illustrate higher levels of moisture and impurity content.
There are several standards classifying coal ranks based on different parameters, in all
of which the coal rank major classifications in ascending order of maturity include
peat, lignite, subbituminous, bituminous, and anthracite [5 8]. The process of coal
rocks maturing from peat toward anthracite is called coalification, in which the vola-
tile and moisture contents diminish, and the carbon content of the rock increases.
Therefore, coals containing low amount of volatile matter and moisture content, and
high carbon content are referred to as high-rank coals.
In coals, the pore structure (specific surface area, size distribution, volume, etc.) is
of great importance, because the matrix pore surface provides the main sorption sites
for the gas. Ji et al. examined the pore structure of some raw coals from anthracite
and bituminous ranks in comparison with their residues [9]. They found out that in
comparing the raw coals with their residues, the coal rank determines the changes in
the size of specific surface areas and micropore volumes. Furthermore, Zhang et al.
conducted an experimental study on coal samples to predict the specific surface fractal
dimension. They concluded that coal rank is among the impressive factors on this
parameter in a coal rock [10]. Hu et al. also proposed that coal rank is one of the
influential parameters on the methane content of the coal [11]. It is also a fundamental
parameter in CO 2 sequestration studies in coals, which will be discussed later in
ECBM section [12,13].
Coals might sometimes be marked with their proximate analysis, which is based
on their organic contents (fixed carbon and volatile matter) and inorganic contents
(moisture and ash). The fixed carbon content of a coal rock increases with coal rank,